There are only a few devices and methods known for non-destructively predicting the strength of a material. There is, however, a need for such devices. A device and method useful for determining the strength remaining in a wooden article, such as an in-service utility pole, is a prime example. There are about one hundred and sixty million wooden utility poles in service. Many states require that utility poles be inspected at least as often as every ten years, and replaced when the strength of the pole drops to below about two-thirds of its design load value. This is an onerous, if not impossible, time-consuming task. This is because determining the strength remaining in utility poles cannot be based solely on a visual inspection, regardless of whether the factors that diminish strength can be seen or not (eg., decay, checks, splits, etc.). Currently, the strength remaining in a utility pole cannot be determined unless the pole is actually broken, which defeats the purpose for the test. A method for testing the strength remaining in wooden articles, without ruining the article, is therefore needed.
Testing methods that do not break the test material but reduce the structural qualities thereof also are disfavored. Certain known techniques do reduce structural integrity. One example of such a method is boring a sample from the pole for inspection. Sample boring promotes fungal deterioration of the wood.
Non-destructive methods have been developed for testing the strength or quality of a material. These methods include X-ray, sonic, electrical resistance, boring and hammering. One disadvantage of most of these methods is that they do not predict strength, but rather require subjective evaluation of the results obtained by the analytical method utilized. For instance, X-ray equipment provides a mass profile that must be adjusted for the circular cross-section of the pole to determine its density. The inspector then must subjectively interpret this profile to ascertain the condition of the pole. At best, the X-ray method may identify void spaces due to decay. Moreover, according to U.S. Pat. No. 4,059,988, which is incorporated herein by reference, X-ray analysis is slow, and therefore is not practical for field use. As a result, X-ray analysis is seldom, if ever, used. And, there currently are no known X-ray methods for predicting the strength remaining in utility poles.
Sonic energy also has been used in attempts to ascertain the strength remaining in wooden poles. The theory underlying the relationship between strength and vibrational attenuation is not entirely understood. However, wooden structures apparently exhibit both wide band attenuation of vibratory energy and selective attenuation within narrow frequency ranges. In general, within the frequency range of about 10-24,000 Hz, the degree of attenuation (acoustic impedance) is related to the strength of the material. More specifically, strength is inversely related to attenuation. Greater attenuation is apparent at higher frequencies. In other words, good wood presents a lower acoustic impedance to low frequencies than to high frequencies. Sonic testing is quick and is therefore more suitable for an initial screening of a wooden article than X-ray procedures.
There are several examples of patented devices that use sonic energy for testing wooden materials. U.S. Pat. No. 3,877,294 describes a device that uses a vibration head to inject a tunable single frequency into a pole at a predetermined point. The vibrational energy is then detected at different points along the pole by a transducer. Comparisons are made between the energy emerging at certain reference points directly opposite the injection point, and energy emerging at other points to determine whether there are voids or regions of rot in the pole.
Another example of a patented device that utilizes sonic energy is U.S. Pat. No. 3,531,983. This device generates an acoustic wave by striking an article with a hammer, which is a method common to a number of known inventions. This procedure does not allow an operator to select the frequency of sound waves that are applied to the wooden article.
In summary, there are a number of techniques that have been developed ostensibly for the determination of material strength. The primary deficiency of all of these devices is the low correlation between predicted and actual strength. Moreover, most known devices do not predict strength, but rather provide some empirical information, such as the time of sonic wave propagation. The interpretation of the output is left to the operator.